A new scuffing test using contra-rotationby Marc Ingram, Clive Hamer, Hugh Spikes

Wear

Text

A new scuffing test using contra-rotation

Marc Ingram a,n, Clive Hamer a, Hugh Spikes b a PCS Instruments, 78 Stanley Gardens, London W3 7SZ, United Kingdom b Imperial College, South Kensington, London, SW7 2AZ, United Kingdom a r t i c l e i n f o

Article history:

Received 24 September 2014

Received in revised form 29 January 2015

Accepted 30 January 2015

Available online 19 February 2015

Keywords:

Lubricated wear including scuffing

Gears

Lubricant additives

Boundary lubrication a b s t r a c t

The mode of lubricant failure known as scuffing provides a significant design constraint in high sliding gears, cams and metal cutting and forming processes. It is therefore important to have an effective test method to measure the scuffing resistance of lubricant formulations. In most existing scuffing bench tests, a moving surface is rubbed against a stationary one at a fixed sliding speed and the load at which scuffing occurs is determined. This approach has two disadvantages. One is that wear of the stationary surface can lead to a large decrease in effective contact pressure during a test. The second is that viscous lubricants often generate significant elastohydrodynamic films at the sliding speeds employed. This means that the scuffing tests measure a complex combination of the influence of the fluid and boundary film-forming properties of the lubricant on scuffing rather than reflecting solely the influence of lubricant formulation.

This paper describes a new scuffing test method in which the two metal surfaces are rubbed together in mixed rolling–sliding with the two surfaces moving in opposite directions with respect to the contact, i.e. in contra-rotation. This enables the sliding speed to be decoupled from the entrainment speed so that the scuffing properties of a lubricant can be determined in boundary lubrication conditions over a wide range of sliding speeds. Also, because both surfaces move relative to the contact, wear is distributed and this minimises changes in contact pressure during a test. & 2015 Elsevier B.V. All rights reserved. 1. Background

Scuffing (often termed scoring in the United States) occurs in lubricated contacts when the lubricating film present in the contact suddenly collapses, resulting in solid–solid adhesion with a consequent very rapid increase in friction and extensive surface damage. It occurs in contacts operating at high pressures and sliding speeds and was first documented in the 1920s when automotive hypoid gears were introduced. This episode led to the development of extreme pressure additives, initially based on lead, sulphur and chlorine, specifically designed to prevent scuffing [1]. Scuffing is still a design barrier in many high-sliding gear configurations, in sliding camfollower systems and in metal-cutting and forming processes.

Numerous test methods have been designed to determine the conditions at which scuffing occurs and thus to measure the scuffing resistance of lubricants and materials and to explore the mechanisms of scuffing. The first scuffing tests such as the four ball method originated in the 1930s and were developed to address the hypoid gear scuffing problem and to assist in the development of extreme pressure additives [2].

One important limitation of the four ball test [3] and the more modern Timken test [4] is that they involve a moving surface rubbing against a stationary counterpart. This generally results in considerable wear on the stationary surface within the contact, which leads to a large increase in effective contact area and thus to a considerable reduction in contact pressure during a test. This means that systems which suffer high wear often require higher loads to scuff than those which have lowwear, a factor that can obscure their intrinsic scuffing resistance. This limitation was recognised in the 1930s, where it was noted that the SAE Extreme Pressure Machine, a disc machine in which both surfaces move relative to the contact, gave more useful scuffing information than tests with one stationary surface [1,5].

It is important to appreciate that scuffing only takes place when all of the protective lubricant films that separate the lubricated rubbing surfaces are destroyed by the rubbing action, i.e. both elastohydrodynamic films due to liquid entrainment and boundary films resulting from lubricant–surface interactions. The penultimate step before scuffing is the removal of the metal oxide film, to expose the nascent metal surface. Since scuffing normally occurs at high sliding speeds, an elastohydrodynamic (EHD) film is often present and the process of scuffing thus involves first the collapse of this fluid

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Wear http://dx.doi.org/10.1016/j.wear.2015.01.080 0043-1648/& 2015 Elsevier B.V. All rights reserved. n Corresponding author.

E-mail address: drmarcingram@gmail.com (M. Ingram).

Wear 328-329 (2015) 229–240 film, then the loss of any micro-EHD films present at asperity conjunctions and finally the destruction of any boundary lubricating films. Any of these three films can provide the critical performance barrier to scuffing. This is reflected in de Gee's “transition diagram” [6,7], as illustrated in Fig. 1. This maps the scuffing response of a specific lubricated system in terms of sliding speed and applied load.

Three transitions are recognised. At low sliding speeds (on the left hand side of Fig. 1), as the load is increased, first the EHD (or microEHD) film collapses (transition from I to II), generally resulting in an increase in friction and wear. Scuffing does not occur however, since a boundary lubricating film is still present. Then as the applied load is increased further, the boundary film collapses at the transition from

II to III and scuffing occurs. At high sliding speeds, (on the right of

Fig. 1), as soon as the EHD film collapses the contact conditions are so severe that any boundary film is also immediately destroyed, so the system passes straight from EHD lubrication conditions to scuffing, i.e. from I to III.